Solar Concentrator Testing Mount Presented by: Sarah Brown Mike Ross

Introduction   About 1000 W/m 2 of solar energy reaches the Earth’s surface.   Cost of conventional solar energy versus cost of fossil fuels.   Multi-junction solar cells versus conventional solar cells.

Dr. Angel’s Project Objective: Make solar energy cheap!   Utilize a parabolic mirror to increase the energy density of sunlight on a single point.   Put multi-junction solar cells at that point to harvest the focused energy.   Create a cooling system to keep cells at focal point as efficient as possible.

Our Project To create a small-scale mirror testing mount that will allow Angel and others to develop the cooling system necessary to dissipate thermal energy from solar cells.

Design Process

Mount Geometry  The paraboloid-shaped Mirror was described by the equation: y = r 2 /6 Not to Scale 0.75 m 1.5 m  We  We were told to hold a glass sphere on the axis 1.5 meter from the vertex (at focus).  We were also told we had to hold a mirror section with its bottom edge 0.75 m from the parabola axis and its top edge 1.5 m from the axis.

Environmental Restrictions   We attached our mirror to the telescope mount in Jane’s Dome on the roof of Steward Observatory.   The small observatory imposed geometric constraints on our mount.  We had to ensure a full range of motion for our mount and figure out how to attach it to the existing mount.

Our Design

Deflection Analysis   Mirror side of beam weighs ~15 lbs.   Focal point side weighs ~15 lbs.   Process: Equations for deflection of a cantilever beam (Table A-9, Shigley) Found no part of the beam would deflect more than.01 in.  Well within specs. Decided to use free weights to adjust for unbalanced ends of arm.

Mount Alignment  Important because mirror will not focus light well if incident light is not aligned to parabolic axis.  We designed a sight parallel to the parabolic axis to allow us to position our mount correctly with respect to the incoming sunlight.  We built a plate with an angled hole drilled in it that can be placed where the glass sphere is normally held. This allowed us to define the parabolic axis.  We used a stick with a mirror and a ruler attached to it to measure the distance from the axis to the top and bottom edges of our mirror to ensure it was aligned properly.

Finished Product

Results and Conclusions

Results   We covered the mirror with paper with evenly spaced holes punched in it.   A piece of paper was held behind the light focus, and the pattern on the paper was observed. On the paper, we were able to see one dot of light for each of the holes cut in the mirror cover.   The uniform pattern observed told us that our mirror was held in the correct shape.   Also, by holding a paper up to the focus, we were able to verify that we had designed our mount such that the mirror focused light to the center of the glass sphere.   Our mount effectively focused light to the glass sphere (providing an excellent scaled simulation of the 13 mirror array).

Why is thermal testing important?  When you use an array of 13 parabolic mirrors, you concentrate an enormous amount of energy into a very small area.  If we ever want solar cells to operate efficiently (or even survive) at the focus of a concentrator, figuring out how to cool them will be absolutely necessary.

Focus Concentration Observing the moon with the complete parabolic array verified that we did, indeed, attain a tight focus. Data from Moon observations fit to normal distributions:

Focus Diameter  To gain a quantitative understanding of the concentration of our focus, we analyzed the moonlight observation data to find the focus diameter.  We defined the focus diameter as the diameter through which 99 percent of the total light energy flux passed.  We found the total light flux through the focus by integrating in the plane of the focus.  Then, we used a computer program to iteratively vary the outer radius of our integration until we found the radius corresponding to 99% energy flux.  We found the diameter to be inches.

Focus Diameter (Cont.)  Concentrating the light hitting the 13 mirrors into a 1.7 inch diameter circle means there is enough energy per unit area to burn through stainless steel in a matter of seconds.  For solar cells to survive the heat generated by the massive amount of energy at the focus, an effective cooling system must be devised to divert that heat.

Conclusions   Thermal testing is absolutely necessary for the efficient production of solar electricity.   Further thermal testing and cooling system design is necessary to attain an appropriate means to cool solar cells.   The mount we designed will provide an effective and convenient means to test those cooling designs.

Thank You Questions?